1. Field of the Invention
The present invention relates generally to the gasification of coal and similar carbonaceous solids and, more particularly, to methods and apparatus for beneficiating the resulting gasification slag.
2. Description of Related Art
Coal and other carbonaceous solids are commonly used as fuel in electricity generating processes. For example, in a fluidized bed process, also referred to as gasification, carbonaceous solids (i.e., coal, char, etc.) are thermally converted into a combustible gaseous product via partial oxidation in a reactive gaseous atmosphere. The product of gasification is a reactive gas composed of carbon monoxide and hydrogen. This gas can be used as a fuel directly or it can be converted to other products such as synthetic oil. The inorganic portion of the coal forms a vitreous slag by-product.
Referring to FIG. 1, there is illustrated a process flow diagram for one embodiment of a gasification facility, such as the Polk Station operated by Tampa Electric Company in Mulberry, Fla. Feed coal is ground and mixed with water at a slurry plant 10 to produce a slurry containing approximately 60-70% solids. The coal slurry is fed into the top of an entrained-flow gasifier 12 along with oxygen from an oxygen plant 14 to produce a high temperature, high pressure, medium Btu synthesis gas of approximately 267 Btu/scf. The synthesis gas is cooled in one or more heat exchangers 16 to generate high-pressure steam that can be used to power a steam turbine 20 to generate electricity. The cooled synthesis gas is cleaned in a scrubber 22a and particulate filter 22b to remove contaminants such as sulfur and ash and then combusted along with nitrogen (N2) in a gas turbine 24 to generate electricity.
Coal gasification differs significantly from combustion, which is a more widely applied coal utilization technology. During combustion, coal is burned to produce heat and fully oxidized combustion gases, primarily carbon dioxide and water vapor. The inorganic portion of the coal forms “fly ash” a very fine dry powdery material that is typically collected dry in cyclones, electrostatic precipitators and baghouses. A much smaller amount of ash also is collected from the combustor as coarser “bottom ash.” Bottom ash particles are irregularly-shaped, frequently porous and low in carbon content.
Not surprisingly, the nature of the by-products generated by gasification and combustion is as different as the technologies are. Slag, which is the by-product of gasification, comprises molten or partially fused particles that come into contact with the furnace wall, become chilled, and solidify. Typically, slag is removed from the gasifier as a slurry. Slag is generally much denser than either fly ash or bottom ash. The coarse component of the slag is essentially a coarse vitreous, high density, abrasive solid that is low in carbon content. The physical shape of the coarse slag particles ranges from rod or needle-like structures to irregular-shaped jagged pieces. The finer slag is comprised of char or unburned carbon particles containing varying amounts of siliceous ash. The carbonaceous phase of the ash is irregularly shaped particles with a highly developed pore structure.
Slag is generally not usable when it comprises a mixture of coarse, vitreous material and fine, porous carbon material, such as when the slag is removed from the gasifier. Accordingly, there is a need for methods and apparatus to beneficiate the slag by-product into its coarser and finer constituents. The method and apparatus should enable not only the economical separation of the slag by-products into useful components, but the separation of the by-products into useful components with acceptable quality and sizing for specific end use markets.